Atomic theory 3 Flashcards
Johann Dobereiner noticed…?
Groups of three elements with similar properties
Name of the group that Dobereiner discovered
Triads
What are 3 examples of ‘triads’?
- Calcium, stroitiam and barium
- Li, Na and K
- Cl, Br and I
How did John Newland list the elements known at the time?
- Order of increasing atomic weight
What did Newland’s discovery only work up until?
16th element
Why was Newland’s discovery important?
Showed that the elements had periodic properties
Name of Newland’s development
Law of Ochtaves
Why does Newland’s discovery not apply today?
- We have noble gases and Newland’s didn’t
How did Mendeleev list elements?
- Order of increasing atomic weight
- Similar properties under eachother
- Gaps for undiscovered (& predicted)
- Reversed order of pairs
Who left gaps for undiscovered elements?
Mendeleev
Who reversed the order of pairs and why?
- Mendeleev
- So that they would be listed in groups of similar properties i.e Te and I
What did Newland’s notice?
That every 8th element has similar properties to the first one.
What did Mosely Henry discover?
Nucleus of each element contained a positive charge of a definite amount = atomic number
After Mosely’s discovery how were the elements arranged?
- Order of increasing atomic number
- transition elements in separate groups
What does today’s table have present that were undiscovered in Mendeleev’s table?
Noble gases
How are elements arranged in the Modern/Mosely’s table?
- Increasing atomic number
How were element’s arranged in Mendeleev’s table?
- Increasing atomic weight
Where are transition elements placed in Mosely’s table (modern)
- In a separate group
Where were the transition elements placed in Mendeleev’s table?
- Included along other elements
- Listed as B sub-groups
Are there gaps in today’s table?
No
Were there gaps in Mendeleev’s table?
Yes for undiscovered elements
What did the Greeks believe (including Pluto & Aristotle)?
- Matter was infinitely divisible
What did Democritus believe?
- Matter is composed of small, indivisible particles.
Who came up with the name ‘atoms’ (atomos)?
Democritus
Where was John Dalton from?
England
When did Dalton propose his meaning theory?
Between 1803-1807
What is Dalton’s theory?
- Matter = small indivisible particles called atoms
- All atoms in an element are identical in mass & properties.
- Atoms cannot be created or destroyed by chemical reactions.
- Atoms combine together to form compounds (different properties from original atom)
What did Dalton’s theory form the basis for?
Much of the work of chemists in the nineteenth century.
Who was William Crookes?
English chemist and physicist
What did Crookes’ carry out in 1855?
Series of investigations into the behaviour of metals heated in a vacuum
What were the 2 steps in Crookes’ experiment?
- Electricity passed through evacuated tubes with two oppositely charged electrodes.
- High voltage of electricity was imposed & produced radiation within the tube
What was observed from Crookes’ experiment?
- Rays of light coming from the negative electrode (cathode) towards the positive electrode (anode).
- Invisible but cast a shadow when an object was placed in their path.
What did Crookes conclude about cathode rays?
- They were made of particles coming from the cathode.
How did the particles from the cathode rays travel?
In a straight line but ‘bent’ by magnetic & electric fields in the manner expected for negatively charged particles.
What was found about cathode rays emitted by different cathode materials?
That they were the same
Who named ‘electrons’?
Johnstone Stoney (1826-1911)
Who was Thomson J.J.?
An English professor of physics
What did Thomson notice in his experiment?
- The beam was attracted towards the positive plate = beam consists of electrons.(negatively charged particles)
What did Thomson’s experiment consist of?
He passed the cathode rays between two oppositely charged plates.
What did Thomson’s experiment allow him to do?
To calculate the ratio of the charge of the electron to it’s mass i.e e/m
Who proposed the ‘plum pudding model’ and when?
Thomson in 1904
Describe the ‘plum pudding model’?
A positive sphere with negative electrons embedded in it in a random fashion.
Who was Robert Millikan?
An American physicist
What experiment is Millikan famous for?
His ‘oil drop’ experiment
What did the ‘oil drop’ experiment allow Millikan to do?
Calculate the size of electrons
What were the steps involved in Millikan’s ‘oil drop’ experiment
- Tiny droplets of oil were sprayed between two charged metal plates
- Oil droplets were forced to pick up electrons to become negatively charged.
What happen when there was no electric field present in the ‘ oil drop experiment’?
The drops fall with a steady velocity
What happened when Millikan placed a positive charge on the top plate?
- the negatively charged oil droplet was attracted to it
- droplet was falling to negatively charged plate due ot gravity
How was the drop’s charge found? (oil drop experiment)
Measurement of voltage needed to bring the drop to rest & rate at which it falls when there is no voltage between the plate.
Who was Ernest Rutherford?
A New Zealand physicist
What is Rutherford famous for?
His ‘gold foil’ experiment
Briefly describe Rutherford’s experiment?
He bombarded extremely thin gold foil with alpha particles (positively charged particles).
What happened to most of the alpha particles in Rutherford’s experiment?
- Passed straight through - undeflected = atoms of gold must be made of empty space.
How were the alpha particles detected (Rutherford’s)?
By hitting off a zinc sulfide screen
What happened to some of the alpha particles in Rutherford’s experiment?
- Passed through but were deflected at large angles
What happened to very few of the alpha particles in Rutherford’s experiment?
- Bounced straight back along their own path
What did the ‘gold foil’ experiment prove?
That there was a very small positive dense core to the atom called the nucleus
What were the positive particles in the nucleus called?
Protons
When did Rutherford do in 1911?
He proposed his nuclear model of the atom
What did Rutherford’s nuclear model of the atom prove?
The the ‘plum pudding’ model was false
Who was James Chadwick?
A past pupil of Rutherford’s
What did Chadwick do?
He bombarded Be foil with alpha particles and neutrally charged particles were emitted from the nucleus.
What did Chadwick call the neutral particles?
Neutrons
What did Chadwick’s work prepare the way for?
The way towards the fission of uranium 235 and towards the creation of the atomic bomb.
What is an alpha particle (L.C)?
Particle consisting of two protons and tow neutrons
Why did Mendeleev place tellurium before iodine in his periodic table of elements? (L.C)
Chemical properties matched (fitted) better in that order
On the basis of the plum pudding model of atomic structure, what results were expected by Rutherford when thin gold foil was first bombarded with alpha particles in 1909? (L.C)
Alpha particles were expected too pass straight through
How did the actual result of Rutherford’s experiment differ from the expected result? (L.C)
- Some alpha particles were deflected at large angles
- A few alpha particles were reflected along original path
How was the model of atomic structure changed as a result of Rutherford’s experiment? (L.C)
- Atoms are mostly made up of empty space with small nucleus
What was the purpose of Millikan’s oil drop experiment of 1908 to 1913? (L.C)
To measure the magnitude of charge on electron
Describe the plum-pudding model of the atom proposed by Thomson around 1897? (L.C)
Electrons in positive sphere (diagram needed)
Give one way of detecting the presence of cathode rays in a vacuum tube. (L.C)
Coloured light/glow
Name the scientist who around 1897 identified cathode rays as subatomic particles and, using a vacuum tube, measured their charge-to-mass ratio. (L.C)
J.J Thomson
What name was given to the subatomic particle that Thomson identified? (L.C)
Electron
State three observations made by Rutherford’s team when they bombarded gold foil with alpha-particles. (L.C)
- Most alpha particles passed straight through
- Some alpha particles were deflected at large angles
- A few alpha particles were reflected along original path
Explain how Rutherford deduced from his results that the nucleus is positive? (L.C)
Repulsion of positive alpha-particles
Explain how Rutherford deduced from his results that the nucleus is small and dense? (L.C)
Statistical analysis of angles of deflection
Describe the structure of Thomson’s ‘plum pudding’ model. (L.C)
Positively charged sphere with electrons embedded in it
What are alpha particles? (L.C)
Particles containing two protons and two neutrons
Explain why some of the alpha particles were deflected at large angles as they passed through the gold foil. (L.C)
Repelled when passing near the nucleus
Why were some alpha particles reflected back along their original paths? Why did this only happen to a very small number of alpha particles? (Rutherford’s ‘gold foil’ experiment) (L.C)
Some - Collided with nucleus
Why - Nucleus is very small
Draw a labelled diagram to show the new structure of the atom proposed by Rutherford. (L.C)
- Draw
- Nucleus shown and labelled with one shell of electrons shown and labelled
Give two properties of cathode ray (L.C)
- Negatively charged
- High-speed
- Negligible mass
What was the basis used by Mendeleev in arranging the elements in his periodic table? (L.C)
- When arranging according to increasing atomic weight there is a periodic occurrence of similar elements
Why did Mendeleev leave spaces in his periodic table? (L.C)
So that similar elements were in the same group
Explain why Mendeleev reversed the order of elements required by his periodic law. (L.C)
To suit properties to groups
State two assumptions of Dalton’s atomic theory of 1808. (L.C)
- Small
- Indivisible
- Identical atomic mass for particular element
Name the scientist who about 1897, measured the ratio of charge to mass of the electron, e/m. (L.C)
Thomson
Name the scientist who about 1910, proved that the electrons in an atom reside in an electron cloud surrounding a small dense positive central nucleus. (L.C)
Rutherford
Name the scientist who, about 1911, measured the charge on the electron, e. (L.C)
Millikan
State two differences between Mendeleev’s table and the modern periodic table of elements. (L.C)
- Left gaps for undiscovered elements
- Reversed some pairs of elements
Name the scientists who used charged oil drops to determine the size of the charge on a sub-atomic particle, and name the sub-atomic particle. (L.C)
- Robert Millikan
- Electron
What two contributions did Henry Mosely make to the systematic arrangement of the element in the periodic table? (L.C)
- Atomic number
- Characteristic positive charge for element
Describe the model of atomic structure which existed immediately prior to Rutherford’s ‘gold foil’ experiment. (L.C)
Positively charged sphere with electrons embedded at random (diagram required for full marks)
In Rutherford’s experiment it was observed that most of the alpha particles went straight through the gold foil. Two other observations were made. State these two observations and explain how each helped Rutherford deduce that the atom has a nucleus. (L.C)
1st observation : deflection of alpha particles
2nd observation : alpha particles reflected
Explain 1st : particles passed close to small, positive mass
Explain 2nd : particles collided with small, very dense mass
Name the scientist who identified cathode rays as subatomic particles (L.C)
J.J Thomson
What contribution did Newlands make to the systematic arrangement of the elements known to him? (L.C)
- Identified periodicity of properties
- Arranged in increasing relative atomic mass
What contribution did Dobereiner make to the systematic arrangement of the elements? (L.C)
- Triads
- Grouped elements of similar properties in groups of three
Name the scientist who identified cathode rays as subatomic particles. (L.C)
J.J Thomson
What contribution did Newlands make to the systematic arrangement of the elements known to him? (L.C)
- Identified periodicity of properties
- Arranged in increasing relative atomic mass
Explain how Rutherford interpreted the results of his experiment to conclude that the atom has a nucleus. (L.C)
- Mass concentrated
- atom is mostly empty space
Describe an experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus. (L.C)
- He bombarded gold foil with alpha particles
- used zinc sulfide detector
- Most passed straight through
- Some deflected at large angles
- Some reflected
